Prior to World War II, sonar,
the technique of sending sound waves through water and observing
the returning echoes to characterize submerged objects, inspired
early ultrasound investigators to explore ways to apply the concept
to medical diagnosis. In 1929 and 1935, Sokolov studied the use
of ultrasonic waves in detecting metal objects. Mulhauser, in
1931, obtained a patent for using ultrasonic waves, using two
transducers
to detect flaws in solids. Firestone (1940) and Simons (1945)
developed pulsed ultrasonic testing using a pulse-echo technique.

Shortly after the close of World War II, researchers in Japan
began to explore the medical diagnostic capabilities of ultrasound.
The first ultrasonic instruments used an A-mode presentation with
blips on an oscilloscope
screen. That was followed by a B-mode presentation with a two
dimensional, gray scale image.

Japan's work in ultrasound was relatively unknown in the United
States and Europe until the 1950s. Researchers then presented
their findings on the use of ultrasound to detect gallstones,
breast masses, and tumors to the international medical community.
Japan was also the first country to apply Doppler ultrasound,
an application of ultrasound that detects internal moving objects
such as blood coursing through the heart for cardiovascular investigation.

Ultrasound
pioneers working in the United States contributed many innovations
and important discoveries to the field during the following decades.
Researchers learned to use ultrasound to detect potential cancer
and to visualize tumors in living subjects and in excised tissue.
Real-time imaging, another significant diagnostic tool for physicians,
presented ultrasound images directly on the system's CRT screen
at the time of scanning. The introduction of spectral Doppler
and later color Doppler depicted blood flow in various colors
to indicate the speed and direction of the flow..

The United States also produced the earliest hand held "contact"
scanner for clinical use, the second generation of B-mode equipment,
and the prototype for the first articulated-arm hand held scanner,
with 2-D images.

Beginnings of Nondestructive
Evaluation (NDE)

Nondestructive
testing has been practiced for many decades, with initial
rapid developments in instrumentation spurred by the technological
advances that occurred during World War II and the subsequent
defense effort. During the earlier days, the primary purpose was
the detection of defects. As a part of "safe life" design,
it was intended that a structure should not develop macroscopic
defects during its life, with the detection of such defects being
a cause for removal of the component from service. In response
to this need, increasingly sophisticated techniques using ultrasonics,
eddy currents, x-rays, dye penetrants, magnetic particles, and
other forms of interrogating energy emerged.

In the early 1970's, two events occurred which caused a major
change in the NDT field. First, improvements in the technology led to the ability to detect small flaws, which caused more parts to be rejected even though the probability
of component failure had not changed. However, the discipline of fracture
mechanics emerged, which enabled one to predict whether a crack
of a given size will fail under a particular load when a material's fracture toughness properties are known. Other laws were developed
to predict the growth rate of cracks under cyclic loading (fatigue).
With the advent of these tools, it became possible to accept structures
containing defects if the sizes of those defects were known. This
formed the basis for the new philosophy of "damage tolerant" design. Components having known defects
could continue in service as long as it could be established that
those defects would not grow to a critical, failure producing
size.

A new challenge was thus presented to the nondestructive testing
community. Detection was not enough. One needed to also obtain
quantitative information about flaw size to serve as an input
to fracture mechanics based predictions of remaining life. The need for quantitative information was particularly strongly in the defense
and nuclear power industries and led to the emergence of quantitative
nondestructive
evaluation (QNDE) as a new engineering/research discipline. A number
of research programs around the world were started, such as the Center for Nondestructive
Evaluation at Iowa State University (growing out of a major research
effort at the Rockwell International Science Center); the Electric
Power Research Institute in Charlotte, North Carolina; the Fraunhofer
Institute for Nondestructive Testing in Saarbrucken, Germany;
and the Nondestructive Testing Centre in Harwell, England.